Time Division Multiple-Access (TDMA) radiotelephone systems which require channel equalization typically impose the need for a linear receiver employing automatic gain control (AGC), conforming to stringent dynamic requirements. Frequently, an additional requirement of the AGC system is to first acquire and then hold the appropriate AGC state for the majority of the timeslot. This permits the receiver to maintain a linear amplitude response which is necessary for optimal channel equalization.
The dynamic requirements for the AGC system can be broken into two cases: the traffic channel (TCH) or the "voice channel", and the random access channel (RACH). In a TCH condition, a given timeslot within the TDMA frame is occupied by the same mobile user for consecutive frames. Neglecting the effects of long-term signal variations, the received power level for a TCH has a tolerably small amount of variation from one TDMA frame to the next. Consecutive timeslots within the frame, however, represent different mobile users, and as such, can be received at widely different levels. For a TCH timeslot, the receiver's AGC system can take advantage of previous knowledge of the appropriate AGC state. It therefore needs to re-configure to that AGC state during a guard period between timeslots.
Unlike the TCH, the RACH can be considered a one-time event. As a result, the base-station receiver does not have the benefit of advanced knowledge of the mobile's received signal level. It must therefore acquire and hold the appropriate AGC state during the actual random access burst. In addition, it must perform this function quickly so as to permit proper reception of the transmitted data and proper channel equalization.
RACH operation is further complicated by the uncertainty of the start of the random access burst with respect to the start of the random access timeslot. In the Pan-European Digital Cellular system, or better known as GSM, the RACH utilizes a shortened TDMA burst called the random access burst. This shortened burst is required to compensate for the time delay between mobile transmission and base reception which when combined with the requirement for the receiver gain to be held constant throughout the majority of the burst, results in the requirement for the AGC system to first detect the mobile's presence on channel and then institute an acquire and hold process. The time period that the receiver uses to institute the acquire and hold process needs to be minimized so that the impact on the radio sensitivity or receiver bit error rate is negligible.
Thus, a need exists for a receiver incorporating an AGC system which detects and applies the appropriate AGC value to a RACH channel in a TDMA system.